The ascending projections from the cochlear nucleus to the inferior colliculus travel in three pathways: a direct pathway from the dorsal cochlear nucleus: a smaller direct pathway from the octopus cell area of the ventral cochlear nucleus: and a ventral pathway which includes direct projections from the ventral cochlear nucleus as well as third order projections from the nuclei of the superior olive. The major efferent projection of the colliculus ascends through the brachium of the inferior colliculus to the medial geniculate body. The colliculus contains interneurons which ramify locally as well as relay cells which project out. We propose to study single cells of the inferior colliculus with the objective of determining both their connections outside the colliculus and the properties of their responses to sound. We will use electrical stimulation of the afferent and efferent fiber tracts of the colliculus to determine from which lower centers individual cells receive input and whether or not they project rostrally to the thalamus. We will also study the discharge patterns of the same cells in response to acoustic stimulation. Knowledge about the connectivity of cells should be of value in trying to infer the functional significance for the hearing process of their response patterns.